Shortages of motor fuels several years ago prompted many efforts toward alternative fuels. Compressed natural gas has been utilized as a fuel, for automotive as well as train locomotive fuel. The large storage volume and the requirement for pressure containment create serious limitations in its use. Using liquefied natural gas or liquefied methane at cryogenic temperatures but at substantially atmospheric pressures permits the carrying of nearly five times as much fuel in the liquefied state as in the compressed gas state at about 2,000 psi (136 bar) with equivalent space requirements. Compared with diesel fuel, the equivalent fuel value of liquefied gas requires about twice the fuel storage volume. In railroad locomotive use, fuel costs comprise typically in excess of 10% of a railroad's operating costs, which provides a strong incentive for highly efficient utilization of fuel. Use of natural gas or substantially pure methane at current price levels can result in fuel cost savings of approximately 10-15% from use of No. 2 diesel fuel. It is anticipated that the price of reformulated, cleaner burning diesel fuels will further increase the price differential.
Most large locomotive diesel cycle engines are supercharged, where the intake air compressor is mechanically driven or turbocharged, with exhaust gases from the engine being expanded through a high speed rotary expander to drive a rotary, centrifugal compressor to compress the incoming air charge to the combustion cylinders. The supercharging or turbocharging of the intake air raises the temperature of the air. This heated air adversely affects the performance of the engine by decreasing the density of the intake air, and therefore limiting the available mass of intake air for a given engine displacement and by increasing the likelihood of detonation of the fuel charge in the cylinders. It has been known to increase the performance of supercharged or turbocharged internal combustion engines by cooling the compressed intake air either after the supercharger or turbocharger or even between the supercharger or turbocharger stages. This cooling is most often accomplished by heat exchange with either a recycled cooling medium such water which then is heat exchanged with an external cooling medium such as air in the case of land-based, stationary power plants or sea water in the case of shipboard power plants or power plants with adequate cooling water supplies. In other instances, the intake air is cooled by heat exchange with surrounding air using a radiator such as a fin and tube heat exchanger. In both these processes, the temperature of the cooled intake air will still be above the temperature of the ambient cooling medium unless additional energy and refrigeration equipment is employed. In the case of a truck, bus, railroad locomotive or stationary engine using ambient air cooling, this intake cooled intake air will be 10.degree. F. to 20.degree. F. (approximately 5.degree. C. to 10.degree. C.) higher than the ambient air temperature. In summer conditions, this may result in an intake air temperature, even after cooling, of 100.degree. F. to 120.degree. F. (38.degree. C. to 49.degree. C.) In other instances, mechanical refrigeration systems have been utilized to achieve controlled cooling of the intake air to desired temperatures substantially independent of ambient temperature conditions. U.S. Pat. Nos. 3,306,032 and 3,141,293 disclose mechanical refrigeration systems for cooling the compressed intake air.
U.S. Pat. No. 4,742,801 describes apparatus for pumping and vaporizing a cold liquefied gas for fuel to a dual fueled internal combustion engine and particularly a diesel engine. However, it does not teach the advantage of using the cold liquefied gas to cool the incoming intake combustion air.
An article by Thomas Joyce in the Spring, 1990 issue of The LNG Observer, Volume 1, No. 1, describes the use of LNG, or liquefied natural gas, as a fuel for an automobile with an LNG vaporizer mounted in the engine compartment of the automobile utilizing engine coolant to heat and vaporize the LNG. It is also suggested that the refrigeration of the LNG could be utilized to cool the incoming air and to thereby, "in essence, supercharge the engine to boost its power." However, although this article alludes to the cooling effect as providing the equivalent of supercharging, it does not deal with the cooling of compressed and heated intake air resulting from the use of a turbocharger or supercharger nor does it teach the controlled aftercooling of the compressed intake air to achieve balanced operation of the vaporizer and aftercooler.